1. Field of the Invention
The present invention relates to an optical cable in which a plurality of optical fibers are assembled into a unit, and the unit is covered with plural pieces of plastic yarn and an outer covering.
2. Description of the Background
For protecting an optical fiber from external pressure and preventing the occurrence of micro bend loss, conventional optical cables generally have a constitution such that a plurality of optical fibers are assembled into a unit and covered with plural pieces of plastic yarn and an outer covering. In optical cables having such structure, plural pieces of plastic yarn are longitudinally applied in a straight form or wound in one direction around the assembled optical fibers. The plural pieces of plastic yarn are cushiony because they are made of a string that is formed by rolling up a thin plastic film having numerous reticulate rips. The optical cables of such structure have been used based on the view that covering the outer periphery of assembled optical fibers or an optical fiber unit with such plural pieces of plastic yarn are useful for protecting the optical fibers from the external pressure and preventing the occurrence of micro bend loss.
In other words, for the purpose of protecting the optical fibers from external pressure it was considered to be sufficient for an optical fiber unit to be entirely covered with plural pieces of plastic yarn in a manner such that the plural pieces of plastic yarn are longitudinally applied in a straight form around the optical fiber unit or wound around it in one direction.
However, in the course of using an optical cable of such structure it was found that occasionally the plural pieces of plastic yarn shrank longitudinally, causing an adverse effect. That is, when the plural pieces of plastic yarn shrink longitudinally, the optical fibers located inside thereof are thereby compressed or caused to shrink, resulting in the optical fibers to meander minutely. This leads to harmful micro bend loss and an increase of transmission loss.
The longitudinal shrinkage of the plural pieces of plastic yarn is considered to occur in the following process. The plural pieces of plastic yarn are stretched by the tension caused in the process of applying them around an optical fiber unit, thereby allowing such elongating strain to be left in the manufactured optical cable. The plural pieces of plastic yarn thus stretched would shrink over time or depending on the variation of ambient temperature after the cable is manufactured. As a result, there occurs an adverse effect such that the transmission loss of the optical fiber changes with time or according to such variation of ambient temperature, rendering the optical cable to have unstable characteristics.
A conceivable method of preventing such adverse effects is to produce a cable without affording such elongating strain to the plural pieces of plastic yarn. However, the plastic yarn used for the above-mentioned purpose is generally made of thin polypropylene yarn and plural pieces thereof are applied around an optical fiber unit such that the weight of a cable as well as its cost is decreased. Such thin polypropylene yarn has a small Young""s modulus and tends to stretch easily. Though thick polypropylene yarn does not tend to stretch easily, there is a limit in terms of allowable volume within a given diameter of the optical cable such that a small number of pieces of thick polypropylene yarn cannot cover an optical fiber unit completely and uniformly. Consequently, the optical fibers are subject to uneven lateral pressure and increased transmission loss. Therefore, a structure using such a small number of thick polypropylene yarns must be avoided. It is advantageous to use thin polypropylene yarn having a small Young""s modulus for such plural pieces of plastic yarn, and in order to manufacture a cable without causing elongating strain, it would be necessary to supply the plural pieces of plastic yarn at an extremely low tension during the manufacturing process of the cable. Such tension must be less than the tension that is given to the optical fibers, and supply-equipment in which such a low tension can be precisely controlled is very high-cost. An optical cable manufactured with such expensive equipment would itself also be very high-cost.
Using other kinds of protective material having a high Young""s modulus would allow a rough tension control.
Such a material, however, would be high-cost, and the optical cable consequently would be expensive.
FIG. 4 is a perspective view showing an example of a conventional optical cable. An optical fiber unit 3 is composed of a tension member 2 and a plurality of optical fibers 1 assembled therearound. The optical fiber unit 3 is covered with plural pieces of plastic yarn 4 assembled thereon. Then, they are covered completely with an outer covering 5.
In order to prevent the bend strain from concentrating on particular optical fibers of the optical fiber unit 3 when an optical cable having such composition is bent, the whole of the optical fiber unit 3 is twisted in one direction. In the example shown in FIG. 4, the optical fiber unit 3 is covered with plural pieces of plastic yarn 4 twisted in one direction. If the plural pieces of plastic yarn 4 shrink longitudinally in the optical cable of such structure, the plural pieces of plastic yarn 4 will thereby have an adverse effect on the optical fiber unit 3 located inside thereof, which will cause each of optical fibers 1 to meander minutely because of longitudinal shrinkage or lateral compression, and consequently harmful micro bend loss will occur.
If plural pieces of plastic yarn 4 are longitudinally applied onto the optical fiber unit 3 in a straight form such as a straight line instead of twined application, the plural pieces of plastic yarn 4 will not laterally compress the optical fiber unit 3. However, even in this case, if the plural pieces of plastic yarn 4 shrink longitudinally, they will have an adverse effect on the optical fiber unit 3, such that the optical fibers 1 will be caused to meander minutely, resulting in increased harmful loss.
As described above, cases were found wherein plural pieces of plastic yarn, which were used to prevent micro bend loss due to the lateral pressure from the outside, shrank and themselves caused micro bend loss. The importance of managing the longitudinal expansion and shrinkage of optical fibers 1 has been recognized in the past. For example, it is described in Japanese Patent Application Laid-Open No. 10-170778, although the purpose is different from that of the present invention.
An object of the present invention is to solve the above-mentioned problem of the conventional technique economically, and to provide an optical cable having stable characteristics such that its transmission characteristics do not exhibit change with time or with temperature. The primary feature of an optical cable according to an embodiment of the present invention is that it is formed by covering the circumference of an optical fiber unit with plural pieces of plastic yarn in a manner such that the plural pieces of plastic yarn are stranded in alternating clockwise and counter-clockwise directions and coarsely bound with a winding string, and then covered with an outer covering.
Here, such clockwise-counterclockwise stranding is a means of twining in which the following stranding operation is repeated; such stranding sometimes being called xe2x80x9cSZ stranding.xe2x80x9d That is, a bundle of linear materials is stranded in one direction and the stranding direction is reversed at a predetermined longitudinal interval, and then the stranding direction is again reversed to the initial direction at the same longitudinal interval. Here, a xe2x80x9cpredetermined longitudinal intervalxe2x80x9d is called the xe2x80x9creverse-turning pitchxe2x80x9d and an xe2x80x9cangle formed by such twining until the stranding direction is reversedxe2x80x9d is called the xe2x80x9creverse-turning angle.xe2x80x9d
There are various means and, ways of clockwise-counterclockwise stranding: in some cases the reverse-turning angle is large such that more than one round of twisting is performed for each reverse-turning, and in other cases the reverse-turning angle is very small such that the stranding is achieved only by meandering slightly.
Another feature of an optical cable according to an embodiment of the present invention is that the reverse-turning pitch of clockwise-counterclockwise stranding of plural pieces of plastic yarn is 100 mm or more but not exceeding 200 mm, and the reverse-turning angle is 180 degrees or more but not exceeding 360 degrees.